1. Field of the Invention
The invention relates to the field of methods and apparatus for determining the validity, denomination and/or similar attributes of conductive tokens such as coins, for operation of check controlled devices. The electromagnetic response of an input token is compared to the response of a reference token, using coils that are offset from a midpoint of the token. Optical means optionally read a code formed on the token, detection of the optical code being related to the timing of two instances when the electromagnetic response of the reference token and the checked token correspond if the checked token is the same as the reference token.
2. Prior Art
Coins and tokens are generally made in distinct sizes for different denominations, and can be sorted by size and/or weight. Coins may also be made in distinct materials or combinations of materials. It is known to discriminate among coins, tokens or the like fed into a coin-feed apparatus, based on the electromagnetic properties of the coins or tokens. Differences in the conductivity and magnetic permeability of different materials are sensed, for example, to determine if a coin is valid, or to trigger diverting means to assist in sorting coins of different denominations. For diverse coins made of the same material (e.g., silver or copper), the size of the coin also affects its electromagnetic properties, permitting discrimination among coins for denominations, for counterfeits, etc.
The apparatus may be responsive to the extent to which a coin passes or attenuates the energy of an incident alternating electromagnetic field, which is a function of the material and the dimensions of the coin. According to one form of coin discriminating apparatus, an alternating electromagnetic field is produced by applying a signal to a coil mounted adjacent the path of the coin, the coil forming part of a feedback oscillator. The effect of the coin on the inductance of the coil circuit can be measured to determine whether the properties of the coin fall within limits expected for a valid coin, for example by monitoring the frequency or phase shift produced. The coin can be examined for the extent to which a current induced in the coin by one coil is coupled, to an adjacent coil when the coin overlaps the two coils. According to a similar form of device, the extent to which a fed coin couples an incident alternating field from an exciter coil to a second, detection coil is used to discriminate among coins and materials. The coin can be examined for the extent to which the coin attenuates the strength of a field between the exciter coil and the detection coil. Examples of such devices are disclosed, for example, in U.S. Pat. Nos. 4,664,244--Wright; and 4,460,080--Howard. Slugs made of ferrous materials produce a strong response in such devices, but non-ferrous materials can also be discriminated, effectively sensing the conductivity and the dimensions of the coin material.
Both the material of the coin and its dimensions affect the electromagnetic properties of the coin. The speed of the coin in passing the discriminating apparatus may also be pertinent. Unscrupulous persons sometimes can defeat such discriminating apparatus, for example, by varying the dimensions and the materials of a slug until a response similar to that of a valid coin is obtained. A foreign coin that is close to the valid coin in size and material also may be substituted. The passage of the coin through the apparatus can be altered such as by slowly lowering the coin on a string until the desired extent of coupling is achieved between the coils (regardless of particular materials), and so forth. It may not be advisable to base discrimination solely on the response of a coin to a particular alternating field. The prior art has attempted various combination tests and other schemes to reduce the incidence of fraud. Such combination tests can involve alternating fields at a plurality of frequencies (U.S. Pat. No. 3,870,137-- Fougere), tests of electromagnetic properties plus coin weight (U.S. Pat. No. 5,085,309--Adamson), etc., in addition to the traditional discrimination by coin size.
One objective of a coin discriminator is to detect small variations between coins, suggesting a need for close tolerances and narrow limits on the range of acceptable coin responses. However, it is also important to have a low rate of false rejections, i.e., rejection of coins that actually are valid coins. Variations in component values, particularly with temperature variation, can cause problems and may require regular calibration, or the inclusion of additional circuit complexity to account for such variations. Any additional expense, and potential requirements for the attention of service technicians due to these problems, are highly undesirable. The coin discriminating apparatus needs to be very inexpensive, durable and operationally effective over the long term without maintenance.
In U.S. Pat. No. 4,469,213--Nicholson et al, a coin discriminator is disclosed that is inexpensive and effective, and is substantially insensitive to component variations, because the mechanism compares the deposited coin's response to the response of a sample coin in a common mode rejection technique. A stack of three coils is provided, with the two outer coils excited by an alternating current having a range of frequency components. A sample coin of the desired denomination is held stationary between the center coil and one of the excited coils, and the deposited coin is dropped between the center coil and the other excited coil. As the deposited coin falls, it passes a point at which the deposited coin and the sample coin produce the same response with respect to the extent to which the outer coils are coupled (through the respective coins) to the middle coil. The respective coils are wired so that the energy from the two outer coils is coupled through the coins to the center coil at opposite polarity. As a result, the sum of the two signals applied to the center coil is reduced theoretically to zero when a deposited coin that is identical to the sample coin passes through the point at which the two coins precisely align in registry between the coils.
The signal from the middle coil is amplified and filtered. A null detector having an adjustable sensitivity (i.e., an adjustable low going threshold) detects the occurrence of the minimum or null. Associated timing circuits require that the null have a particular maximum and minimum duration in order to trigger the output, thus obviating most attempts to defeat the apparatus using a coin on a string. One benefit of using a comparison between the deposited coin and a sample coin is that the same device can be changed to test for a different coin denomination by changing to a new sample coin. However, the size of the coin affects that rate at which the coin falls through the device, and the rate of the fall affects the duration of the null. To accommodate coins of different sizes, Nicholson teaches a spring loaded pivoting coin ramp leading into the detection zone between the coils. A heavier coin moves the coin ramp to a steeper angle than a lighter coin. Thus, smaller coins pass more slowly than larger ones, and the duration of the null remains approximately the same. A slide arrangement is provided to position the coil stack and the sample coin where required to align with a deposited coin of the denomination to be checked. A solenoid-driven coin diverter is mounted downstream of the coil stack, and is triggered by the null detector to either pass or divert the deposited coin.
According to Nicholson, the axis of the coil stack is placed at the midpoint of the sample coin in the direction of coin travel. Only one null on the signal from the middle coil occurs during passage of the deposited coin, namely when the deposited coin and the sample coin are in registry with one another between the detector (middle) coil and the respective exciter (outer) coils.
The Nicholson device is advantageous in that it is durable and relatively simple. However, it relies heavily on the duration of the null that occurs at the point when the sample and deposited coin align, which is not completely effective. The person or mechanism depositing the coin may have some control over the rate at which the deposited coin passes the detection zone. The spring loaded coin ramp tends to slow operation. The ramp is a moving part whose operation may vary over time, and may require maintenance.
According to the present invention, it has been discovered that by placing the coil stack at a point that is offset from the midpoint of the coins along the feed direction, leading or trailing, it is possible to produce and examine two nulls occurring as the deposited coin passes the sample coin. One null may occur when the sample and deposited coins are in registry, as in Nicholson. Another null occurs when the two coins are symmetrically arranged relative to the offset coil axis. The character of the two nulls is distinct in certain test situations, such as when the sample coin and the deposited coin are of different materials and/or sizes. The spacing of the two nulls in time is a measure of the rate of fall of the coin, making a coin speed adjustment mechanism unnecessary. The signal is examined not only for the low-going threshold at the nulls, but also can be examined for the height of a peak occurring between the nulls. Preferably, the AC signal from the detection coil is rectified, filtered and digitized for processing via a microcomputer. An optical encoder can be associated with the device, for reading minted identification codes, provided on the coin at a predetermined point that can be windowed in time by the microprocessor, with reference to the signal level from the detection coil. The device is more accurate and less dependent on moving parts than the known, single-null device of Nicholson, and is very inexpensive to produce and to use.